1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) receiving system, and more particularly to an apparatus for correcting a frequency offset caused by a difference between transmitted and received frequencies in OFDM receiving system within 1/4 range of carrier frequency interval.
2. Description of the Prior Art
In a conventional wireless communication channel and digital high definition television (HDTV) transmission channel, an Inter-Symbol Interference (ISI) is generated by an occurrence of multi-path fading in a received signal. Particularly, when high speed data for HDTV are transmitted through the channel, the ISI increases causing errors to be generated during the recovery of the high speed data at receiving end. To solve this problem, recently, OFDM technique has been implemented to take an appropriate measure to counter against the multi-path fading problem in the Digital Audio Broadcasting (DAB) and HDTV standards. OFDM technique converts serially-inputted symbol streams with N number of symbols, multiplexes the N number of parallel symbols into different subcarrier frequencies, adds all the mutiplexed data, and transmits the added data. Here, the N number of parallel data are defined as one unit block, and each subcarrier of the unit block has an orthogonal characteristic, which does not have an influence on inter-subcarrier channels. Compared to a conventional carrier transmission method, OFDM method can decrease the occurrence of ISI caused by the multi-path fading while maintaining the same symbol transmission rate and increasing symbol period as much as by the number of subcarrier channels (N). Especially, OFDM method utilizes an insertion of guard interval (GI) between the transmitted symbols for an ISI reduction, making it possible to realize a simplified structure of channel equalizer desired. In contrast to a conventional Frequency Division Multiplexing (FDM) type, OFDM method has a characteristic that spectrums of each subcarrier channel are superimposed enabling it to have a higher spectrum efficiency. Further, the spectrum has a waveform of a rectangular shape and an electric power is uniformly distributed at each frequency band, which prevents from being affected by the same co-channel interference signal.
Meanwhile, an abrupt variation of a channel characteristic makes the frequency band of transmitting signal susceptible to the Doppler Effect, or an unstable state of a tuner causes non-synchronization between transmitting and receiving frequencies resulting in frequency offset. The frequency offset decreases the decoding capability of the receiving system by varying the phase of receiving signal. In an OFDM method using multi-carrier, a symbol according to each sub-channel is detected. In presence of frequency offset, an orthogonal characteristic between each subcarrier frequency is not maintained causing an interference between two adjacent sub-channels. Particularly, each subcarrier is closely distributed within a band as the number of OFDM sub-channel increases, such that the interference between adjacent sub-channels is generated even at a small frequency offset value. Hence, the frequency offset correction is one of the most important things to consider when designing and constructing an OFDM receiving system.
Concerning this frequency offset correction, in 1995, F. Daffara and O. Adami proposed a frequency offset correction circuit using a guard interval comprised in an OFDM signal in "A new frequency detector for orthogonal multi-carrier transmission technique" disclosed in Prc. of VTC'95. In this prior art, a characteristic curve related to the frequency offset obtained in respect to the guard intervals and circular prefix characteristic of the transmitting signal, is utilized to construct a frequency offset correction circuit using phase locked loop (PLL).
The circuit has merits of a simple structure, but it has a drawback in that as the frequency offset value increases, its correcting ability decreases. More, the circuit can not be applied when the frequency offset value is larger than the frequency interval between subcarriers.
In 1994, P. H. Moose proposed a frequency offset correction circuit without using PLL, which efficiently adapts to channel environment by estimating a frequency offset value through transmitting a special symbol stream in frequency selective fading channel, in "A technique for orthogonal frequency division multiplexing frequency offset correction" IEEE Trans. Commun., vol. COM-42, published on October, 1994.
However, in this method, the transmitting speed of useful symbols decreases as the symbol stream is transmitted repeatedly, and its application is limited when the frequency offset (.epsilon.) is larger than the frequency interval between the adjacent subcarriers.
That is, when the frequency offset (.epsilon.) is .vertline..epsilon..vertline.&gt;0.5, the offset correcting capability of the receiving end decreases since the frequency offset causes a circular shift between decoded OFDM symbols by deflection-shifting frequencies of each subcarrier to an adjacent sub-channel bandwidth. Here, only the circular shift is generated if the frequency offset value is an integer, whereas both a cross talk, an adjacent channel interference, and the circular shift is generated if the frequency offset value is not an integer since the received signal is defined as a discrete signal in Fourier operation.
When the frequency offset (.epsilon.) is .vertline..epsilon..vertline.&lt;0.5, only the cross talk is generated without the circular shift. When the frequency offset is an integer (.epsilon.=m, where m is an integer), although the interference between a corresponding channel and its adjacent channel is not generated, an 1-th symbol is transmitted to the (1-m)-th sub-channel. Therefore, in a system utilizing a general correction method used for the frequency offset, an accurate recovery of transmitted signal is limited.
Accordingly, for an accurate frequency offset correction, at first, a process for reducing the frequency offset value within the range which does not generated a circular shift (.vertline..epsilon..vertline.&lt;1/2) is performed, and next, a frequency offset detection process for accurately detecting the frequency offset is performed.
For the above described method, in 1994, F. Classen and H. Myer proposed a frequency offset estimation method using two steps, an acquisition process for reducing initial frequency offset within a constant range and a tracking process for tracking accurately the frequency offset before estimating the frequency offset value, in "Frequency synchronization algorithm for OFDM systems suitable for communication over frequency selective fading channels" published in Proc. Of VTC'94 in 1994.
However, this method has a problem in that the amount of calculation for the acquisition process is large and the frequency offset does not decrease according to the channel characteristics.
That is, although the frequency offset value is decreased within 1/2 range of carrier frequency the amount of calculation for the tracking process increases if the frequency offset value is near the boundary of 1/2 range of carrier frequency interval.